The widespread use of window systems in personal computers and workstations has resulted in a demand for larger monitors so as to be able to represent more windows on a monitor. Although more than 14-inch monitors have become increasingly common recently, this option is exceedingly expensive, since even a relatively small enlargement of the monitor area leads to a considerable increase in the cost of the monitor. A user of window systems requires e.g. for simultaneous working in a text processing system and e.g. in a graphics program or a CAD program several windows on a screen. Because of the size of a screen it is nearly impossible to represent all the necessary windows simultaneously and e.g. to activate one or other of the windows by means of a keyboard or a mouse. The user is thus forced to close some windows and open others when he wishes to change from one application to another. In applications requiring a frequent back and forth between the various windows which represent different applications, the perpetual closing and opening is a tiresome activity, which, for one thing, markedly reduces the user's working efficiency.
For this reason the need arose to connect two or more screens to a personal computer or a workstation and to place them next to each other so as to effectively double the effective line width through the use of two monitors. As a result a user is able e.g. to assign Windows control windows to one monitor, while an application, such as e.g. a text processing, is represented on the other monitor. If a user wants to incorporate graphic images in a text processing, a system with two monitors enables him to perform a text processing on one monitor while on the second monitor a graphic image is manipulated and dimensioned so as to fit into the place reserved for it in a document which is being worked on. The same also applies e.g. to CAD applications, where there is a desire for a high-quality, high-resolution screen for the CAD application, whereas the control symbols on the other screen do not require the use of such a high-quality screen, thus enabling a saving in working place costs.
In order to control two or more monitors from a computer so as to double or more than double the effective line width, it is necessary to “distribute” the graphic data generated by the computer correctly to the individual screens. Among the devices for storing data to be represented on monitors are the so-called video random access memories or VRAMS, in which the data for display are addressed linewise. A line of a video random access memory can thus contain as many pixels as can be represented in one line on an assigned monitor, this line only having to be addressed with one address, however, as is known to persons skilled in the art.
In a known system for controlling two monitors, each monitor is equipped with its own digital-analog converter, which is needed to convert the digital data delivered by the computer into analog data required by the monitor. Furthermore, each digital-analog converter is connected to its own video random access memory, which is in turn controlled by its own graphics control. In a known system the data for display delivered by a computer are passed over a host bus to a buffer, undergoing a software-implemented image division on the way. The host CPU, i.e. the central processing unit of the host system, stores the data to be represented on the two monitors in video random access memories, one for each of the monitors, from which, by means of the graphics control provided for each video random access memory, data can be read out and converted into an analog form for display on the monitors. The known system thus comprises two parallel branches, a branch consisting of a monitor, a digital-analog converter, a video random access memory and a graphics control, the two branches being connected and controlled by a software image division unit, which is fed by the host bus of the computer via a data buffer, which may likewise be implemented as a VRAM. The host CPU must thus undertake the division of the pixel stream in this known system. The pixel stream to be represented on the monitors must thus be apportioned to the respective video random access memories by the software image division unit.
A disadvantage of the known system is that a separate video random access memory and a separate graphics control must be provided for each monitor, which means that a system of two monitors and a computer is unnecessarily costly. Furthermore, the software-implemented image division in the software image division unit is slow, meaning that the image buildup times of the system of two monitors are increased unnecessarily.